Two balls P and Q are thrown vertically upwards simultaneously from the ground with velocities $20\ \text{m s}^{-1}$ and $35\ \text{m s}^{-1}$ respectively. The distance between the two balls when the velocity of P becomes zero is: (g = $10\ \text{m s}^{-2}$)}
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Always compute positions at the same time instant for relative distance problems.